Merlin is an open-cycle rocket engine burning RP-1 (highly refined aviation kerosene) and liquid oxygen. For SpaceX, it is perhaps the most important engine, since it drives their rockets from the very beginning – from Falcon 1 through the current Falcon 9 to the Falcon Heavy. SpaceX manufactures up to several hundred of these engines a year, and is therefore also the largest manufacturer of rocket engines in the world. Merlin has gone through many changes over the years, and there are several different variants that differ greatly. At the moment, the company uses Merlin 1D (shown) on the first stage of Falcon 9 and the 1D-Vacuum variant on the second stage (modified engine version optimized for vacuum operation).
Merlin 1D has the best Thrust-to-weight ratio (TWR) of all rocket engines in the world. It weighs 470 kg and the latest version has a 845 kN thrust on the sea. Merlin’s great advantage is its re-startability (it can easily be turned off after ignition and then re-ignited), allowing you to perform test spells before starting, while allowing a first-stage missile landing, which requires several engine sparks. Another advantage is regenerative cooling, which makes the engine easy to reuse. The principle of regenerative cooling is that the very cold fuel before burning in the chamber first passes the channels in the walls of the engine’s expansion nozzle and thereby cools it continuously.